Slurry phase polymerisation of olefins is well known wherein an olefin monomer and optionally olefin comonomer are polymerised in the presence of a catalyst in a diluent in which the solid polymer product is suspended and transported.
This invention is specifically related to polymerisation in a loop reactor where the slurry is circulated in the reactor typically by means of a pump or agitator. Liquid full loop reactors are particularly well known in the art and are described for example in U.S. Pat. Nos. 3,152,872, 3,242,150 and 4,613,484.
Polymerisation is typically carried out at temperatures in the range 50-125° C. and at pressures in the range 1-100 bara. The catalyst used can be any catalyst typically used for olefin polymerisation such as chromium oxide, Ziegler-Natta or metallocene-type catalysts. The product slurry comprising polymer, and diluent, and in most cases catalyst, olefin monomer and comonomer can be discharged intermittently or continuously, optionally using concentrating devices such as hydrocyclones or settling legs to minimise the quantity of fluids withdrawn with the polymer.
The loop reactor is of a continuous tubular construction comprising at least two, for example four, vertical sections and at least two, for example four, horizontal sections. The heat of polymerisation is typically removed using indirect exchange with a cooling medium, preferably water, in jackets surrounding at least part of the tubular reaction loop. The volume of the loop reactor can vary but is typically in the range 20 to 120 m3 the loop reactors of the present invention are of this generic type.
Maximum commercial scale plant capacities have increased steadily over the years. Growing operating experience over the last few decades has led to operation of increasingly high slurry and monomer concentrations in reaction loops, the increase in slurry concentrations has typically been achieved with increased circulation velocities achieved for example by higher reactor circulation pump head or multiple circulation pumps as illustrated by EP 432555 and EP 891990. The increased velocity and head requirement has led to increasing energy consumption as slurry concentrations increase. Despite increased operating experience the volume of individual polymerisation reactors has also needed to be increased to accommodate the desired production capacity. Construction and commissioning of new commercial plants is very expensive and therefore new designs seek to achieve any required scale-up in capacity whilst changing parameters that present minimum risk to the successful operation of the new unit. Typically reactor loop volume has been increased by adding legs and/or length to existing reactor loops or even by linking two existing loops together whilst maintaining the reactor loop internal diameter at about 24″ (600 millimeters) or below. The increase in reaction loop volume by increasing length at a fixed diameter leads to steadily increasing absolute (and even specific) loop pressure drops (and therefore power consumption).
Increasing the diameter of commercial scale reactors to increase reactor volume has been seen as giving greater scale-up risk than that associated with increasing length. The increased risk has been associated with concerns over maintenance of good thermal, compositional and particle distribution across the reactor cross-section without excessively increasing turbulence (e.g. circulation velocity) and associated pressure drop/power in the polymerisation loop. Inadequate cross-sectional distribution could lead to increased fouling, reduced heat transfer and reduced polymer productivity and homogeneity.
In addition, reactors are typically designed and constructed with a constant internal diameter around the entire loop, except for example where fittings, such as the circulation pumps, dictate a different (larger or smaller) diameter at a specific location for a particular reason. There would have been an expectation that varying the internal diameter between for example the vertical and horizontal sections would lead to fouling problems. We have found that this is not the case.